Field of the Invention
The invention relates to a measuring head, in particular for use in the measurement of a fuel rod of a fuel assembly and/or a spacer in a fuel assembly of a nuclear engineering installation. The invention also relates to a measuring device having a measuring head of this type and to the implementation of a measuring device of the type.
To generate electrical energy in a nuclear installation, in particular in a nuclear power plant, it is customary for fissile material to be subjected to controlled nuclear fission. The fissile material is held in a number of fuel rods, in which the material, for example in the form of pellets, is surrounded by suitable cladding tubes. A plurality of fuel rods of this type are usually combined to form a fuel assembly. It is thereby ensured, even in long-term operation of the nuclear engineering installation, that a predetermined geometry is maintained and in particular that specified distances between the fuel rods of a fuel assembly are maintained by means of suitable spacers.
When a nuclear engineering installation of this type is operating, the respective components or installed parts are subject to ageing processes, such as for example corrosion. Furthermore, components of this type are also exposed to radioactive radiation over a prolonged period of time, which may in addition to the standard ageing-induced corrosion or fatigue effects also lead to changes in shape or dimensions. This may in turn have an adverse effect on the performance or reliability of the component in question. Particularly when using fuel rods, ageing-induced effects of this type can lead to the phenomenon known as creep, in which the fuel rod in question changes, inter alia, its diameter. Furthermore, however, it is also possible for oxidation to occur at the surface of a fuel rod, as a type of corrosion, which may lead to the formation of a more or less continuous oxide layer on the surface of the fuel rod. Depending on the thickness of the oxide layer which forms, the wall thickness of the cladding tube below may be adversely affected, even to such an extent that it drops below a limit wall thickness which is still classified as acceptable for the fuel rod to continue to operate. Both effects, i.e. the changes in shape or dimensions, on the one hand, and the formation of oxide layers, on the other hand, may therefore together or even individually cause the reliability and correct functioning of the fuel rod in question to be adversely affected.
For this reason, as part of regular maintenance, it is customary to test installed parts in a nuclear engineering installation for the occurrence of such effects. In this context, the term “installed part” is to be understood as meaning in particular a fuel rod, a fuel assembly channel, a spacer or some other structural part in the nuclear engineering installation. When carrying out a check of this type, on the one hand a measurement is carried out on a random sample of the installed parts to determine whether a significant change has occurred in the fuel rod diameter, also, the corresponding installed parts are subjected to a random sample measurement to check for changes in shape and dimensions. Specially produced sensors, which can be used to acquire sample measurement data for selected installed parts, in particular selected fuel rods, can be used for these maintenance and test measurements.
Independently of this, on the other hand, a sample examination is carried out on the installed parts at regular maintenance intervals to determine whether and to what extent oxide layers have formed. These measurement results are used to draw conclusions as to the ageing state of these components, it being possible in particular to replace some or all of the fuel rods as required. Specifically designed measuring devices, in which a suitable measuring probe is used to determine measurement data concerning the layer thickness of the oxide layer on selected fuel rods or other installed parts, are likewise employed to determine the corresponding measurement data.
However, it is still a relatively complex matter to determine information concerning the ageing state of the corresponding installed part, in particular the fuel rods or spacers, in this way. In particular, the above-mentioned measurements may entail down times in the nuclear engineering installation, and such down times need to be kept particularly short if only for economic reasons. To maintain relatively short down times, it may be necessary for the sample measurements to be restricted to a relatively small number of the components which are to be measured, in particular fuel rods or spacers, and consequently the database available for reliable assessment is relatively small. This in turn may lead to unacceptable inaccuracy when determining the state of the fuel elements and in particular forecasting the future ageing-induced performance of the fuel assemblies.